In the following, the invention is described on the basis of a tractor, whose toolbar (hitch) can be used for fitting different attachments. This could, for example be a plough, a harrow, a mower or another tool. However, the invention is not limited to a tractor being the vehicle. Also other vehicles can have attachments, for example street sweepers, forestry machines etc.
During operation, the attachment should be set to a position, that is, a height, which corresponds to the application purpose. For example, a plough, which is fixed on a tractor, should be able to penetrate to a certain depth into the earth. A too large or a too small depth in not wanted. However, the set position is not maintained for the whole operation duration of the tractor. For example, the plough has to be lifted, when the tractor leaves the field to drive to another field. The plough also has to be lifted, when the tractor changes its driving direction, however, the blocks should be thrown in the same direction. After the lifting, the plough has to be returned to the previously set position.
The combination of a hydraulic cylinder and a control device forms part of an electro-hydraulic position control. When, during lifting, the amount of hydraulic fluid supplied to the cylinder is controlled, a substantially load independent lifting movement can be realised. The speed of the cylinder in the lifting direction can be kept substantially proportional to a control signal. The maximum speed of the cylinder is thus proportional to a maximum desired value of the control signal, and the amplification is constant and independent of the load.
When lowering the load, that is, the attachment, however, it is different. Particularly with single-acting cylinders, the lowering movement can usually not be made load independently. Attachments with a higher weight sink faster than attachments with a lower weight. This leads to variants in the rule amplification in connection with the position control for varying loads.
From U.S. Pat. No. 4,846,283 is known a vehicle, with which the lowering speed of the attachment can be braked. To a certain limit, a control signal and thus the maximum deflection of the valve can be adjusted. This applies for a predetermined load. When, however, the load changes, a new adjustment or setting is required.
U.S. Pat. No. 6,016,875 describes a control, which sets the amplification iteratively, which is proportional to the desired lifting speed. The process shown is based on an overshoot and requires some iteration steps. During this iteration, the driver may have an unpleasant feeling, when the acceleration changes.
U.S. Pat. No. 5,320,186 describes a method of achieving a constant and load-independent maximum speed when changing the position of the attachment. This method uses a speed control, which differentiates the position of the attachment numerically, in order to detect its speed. This speed is compared with a desired speed, in order to detect a speed error, which is then supplied to an integrator. Thus, this control substantially corresponds to a proportional position error control having a ramp function as reference. The gradient of the ramp is the desired speed.
In controls with a simple design, this method may lead to too high speeds. In controls, situations may occur, in which the position control and the speed control work against each other, which involves the risk that too high speeds occur. Additionally, too high accelerations may occur. On the one hand, these may cause damage. On the other hand, when lifting the load, the front wheels may lift off from the ground, which can lead to dangerous situations.
Based on the foregoing, it is the general object of the present invention to improve the position control of the attachment.